This invention relates to an improved operating system and oligomerization technique for upgrading light olefins to liquid hydrocarbons. In particular, it provides a semi-continuous reactor system for producing distillate range fuel products by oligomerizing olefinic feedstock to produce distillate product for use as diesel fuel or the like. It provides a technique for oligomerizing lower alkene-containing light gas feedstock, optionally containing ethene, propene, and/or butylenes, to produce predominantly C.sub.10.sup.+ distillate hydrocarbons and a minor amount of olefinic gasoline and other useful products.
Developments in zeolite catalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5.sup.+ gasoline, diesel fuel, distillates, lubricant stocks, etc. In addition to basic chemical reactions promoted by ZSM-5 type zeolite catalysts, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.4 alkenes. Conversion of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al (U.S. Pat. No. 3,845,150) to be effective processes using the ZSM-5 type zeolite catalysts.
In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially ethene, propene and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5.sup.+ hydrocarbons. Product distribution for liquid hydrocarbons can be varied by controlling process conditions, such as temperature, pressure and space velocity. Olefinic gasoline (eg, C.sub.5 -C.sub.9) is readily formed at elevated temperature (e.g., up to about 350.degree. C.) and moderate pressure from ambient to about 5500 kPa, preferably about 250 to 2900 kPa. Under appropriate conditions of catalyst activity, reaction temperature and space velocity, predominantly olefinic gasoline can be produced in good yield and may be recovered as a product or fed to a high pressure reactor system containing high acid acitivity catalyst for further conversion to heavier hydrocarbons, especially C.sub.10 -C.sub.20 distillate-range products. Distillate mode operation can be employed to maximize production of C.sub.10.sup.+ aliphatics by reacting the lower and intermediate olefins at high pressure and moderate temperature. Operating details for typical olefin oligomerization units are disclosed in U.S. Pat. Nos. 4,456,779; 4,497,968 (Owen et al.) and U.S. Pat. No. 4,433,185 (Tabak), incorporated herein by reference.
At moderate temperature and relatively high pressure, the conversion conditions favor production of distillate-range product having a normal boiling point of at least 165.degree. C. (330.degree. F.) Lubricants can be manufactured by multistage reactors according to U.S. Pat. Nos. 4,520,215 and 4,568,786 (Chen and Tabak). Lower olefinic feedstocks containing C.sub.2 -C.sub.4 alkenes may be converted selectively. While propene and butene may be converted to the extent of 50% to 99% at moderate temperature, only about 10% to 30% of ethene will be converted using only HZSM-5 or similar process conditions and acid zeolites.
It has been found that olefinic light gas rich in C.sub.3 -C.sub.4 alkene can be upgraded to intermediate dimer and trimer liquid hydrocarbons rich in heavier C.sub.6 -C.sub.12 olefinic hydrocarbons by catalytic conversion in a fixed bed or turbulent fluidized bed of solid acid zeolite catalyst at a high space velocity and under controlled reaction severity conditions to prevent substantial formation of aromatics. This technique is particularly useful in a two-stage process for upgrading olefinic components of LPG and FCC light gas. Typical feedstock may contain significant amounts of ethene, propene, butenes, C.sub.2 -C.sub.4 paraffins and hydrogen produced in cracking heavy petroleum oils or the like. It is a primary object of the present invention to provide a novel technique for upgrading such lower olefinic feedstock to heavier lubricants, distillate and gasoline range hydrocarbons in an economic multistage reactor system.